Light emitting diodes (LEDs) are used as flash light sources for cameras, especially cameras found on multifunction portable devices. The LEDs may include High Brightness LEDs (HBLEDs) that provide a bright light source especially beneficial for photography.
Maximizing both the lifetime and luminous flux output of the LED requires balancing the competing characteristics of high current for higher luminous flux and less current for lower LED junction temperature. LEDs have a maximum junction temperature that results in a specific lifetime. Accordingly, this maximum junction temperature leads to the current, voltage, and on time that may be used to operate the LED. The LED printed circuit board mounting in part determines the thermal resistance of the LED junction to ambient temperature. The composite thermal resistance then translates the applied power, i.e., electrical applied power minus optical power emitted, into a LED junction temperature.
The applied power is equivalent to the LED forward voltage multiplied by the operating current. The power that is translated into heat is that portion of the applied power that is not converted to emitted light.
For a given large sample size of LEDs, the LEDs exhibit large varying forward voltages for identical operating currents. Consequently, the junction temperature for the LEDs will be substantially different. Even if all LEDs exhibited the same forward voltage and were operated using the same currents, the junction temperature at the end of a flash period may still be different. There are several reasons for this.
One source of difference is the difference in optical efficiency between LEDs when converting applied power to luminous flux. A second cause of the temperature difference is the difference in thermal resistance for different mounted LEDs. A third cause results from the starting junction temperature being different at the start of any given operation period. And a fourth cause is a result of LED aging whereby the luminous flux output (optical power emitted) for a fixed operating current lowers with lifetime.
Of the four causes listed above, optical efficiency and aging can generally be ignored in flash applications. This is a result of the LED manufacturers binning their devices for matching luminous flux (i.e., optical efficiency) and the small operational lifetime of the installed LED (e.g., smartphone replacement cycle is substantially shorter than LED lifetime).
Variations in thermal resistance are dominated by the variations in the case to ambient thermal resistance because this term is the largest. Luckily, the composite thermal resistance is substantially constant and exhibits a variation of perhaps ±/−5%.
Therefore, junction temperature variation is predominantly a function of variations in forward voltage and starting junction temperature.
Ignoring the variations in LED forward voltage, which can easily be +/−30%, the starting junction temperature can be the most variable cause of LED maximum junction temperature. This is a result of the large ambiguity in starting temperature at the beginning of a flash cycle. For example, a smartphone ambient temperature may be from 0° C. to 50° C. and higher, and if the LED is operated several times in succession, the average junction temperature begins to rise even higher than that of the ambient. These situations may collapse an initial and controlled factory temperature span of 125° C. (i.e., from 25° C. to 150° C. LED junction temperature) to an operational field span of 65° C. (i.e., 85° C. to 150° C. LED junction temperature).
Further, because of the wide variations in forward voltage across LEDs, designers may use a 50% design margin in order to prevent the LED junction from overheating and thus damaging the LED. This results in reduced LED output and performance.